课程名称︰计算机网路 课程性质︰系必修 课程教师︰逄爱君 开课学院：电资学院 开课系所︰资工系 考试日期（年月日）︰2010.6.23 考试时限（分钟）：180 是否需发放奖励金：是 （如未明确表示，则不予发放） 试题 : Question 1:"Quickies"(30%) Answer each of the following questions briefly, e.g., in at most a few sentences. a)(5%)Where can queueing occur in a router? Briefly explain the conditions that lead to such queueing. Queueing can occur at both the input ports and the output ports of a router. Queueing occurs at the ourpur port when the arriving rate of packets to the outgoing link exceeds the link capacity. Queue occurs on and input port when the arriving rate of packets exceeds the switch capacity; head-of-the-line blocking can also cause queueing at the input ports. b)(5%)What is HOL (Head-of-Line) blocking? Does it occur in input ports or ouput oirts? The switch fabric can only switch the packets at the head of the buffer per clock, HOL arises in input ports when packets arriving at different input ports are destined for the same output port. If the HOL packet of a certain buffer at the input cannot be switched to an output port because of contention, the rest of the packets in that buffer are blocked by that Kead-of-Line packet, even if there is no contention at the destination output ports for those packets. c)(5%)What is meant by the term "tunneling"? In tunneling, an IP datagram (e.g., an Ipv6 datagram) is carried as the patkiad if another IP datagram. d)(5%)Briefly describe TCP's slowstart algorithm. What causes the TCP slowstart algorithm to end, and TCP congestion avoidance begin? When connection begins, TCP's congestion window is 1 MSS. For each ACK received, the congestion window size is double. This causes the number of segments sent per RTT to increase exponentially. Slowstart ends when a threshold window size is reached. e)(5%)Two hosts simultaneously send dat through a link of capacity 1Mbps. Host A generates data with a rate of 1Mbps and uses TCP. Host B uses UDP and transmits a 100bytes packet every 1ms Which host will ontain higher throughput? f)(5%)What are different inter-AS and intra-AS protocols used in the Internet g)(5%)What is the difference between a group-shared tree and a source-based tree in the context of multicast routing? Problem 2: Congestion Control and TCP (40%) a)(5%)What is the difference between congestion control and flow control? Flow control is about matching the speed of a sender to the capabilities of the receiver. Congestion occurs when senders overutilize the resources within the network. b)(5%)It is said that a TCP connection "probes" the network path it uses for available bandwidth. What is meant by that? TCP keeps increasing its send rate (by inceasing its window size) until loss occurs (at which point congestion has set in). TCP then sets its rate lower but again begins increasing its send rate to again determine the point at which congestion sets in. In this sense, TCP is contantly probing the network to see how much bandwidth it can use. c)Suppose that in TCP, the sender window is of size N, the base of the window is at sequence number x, and that the sender has just sent a complete window's worth of segments. Let RTT be the sender-to-receiver-to-sender round trip time, and let MSS be the segment size. i.(10%)Is it possible that there are ACK segments in the receiver-to- sender channel for segments with sequence numbers lower than x? Justigy your answr. ii.(5%)Assuming no loss, what is the throughput(in packets/sec) of the sender-to-receiver connection? iii.(5%)Suppose TCP is in its congestion avoidance phase. Assuming no loss, what will the window size be after the N segments are ACKed? N+1 d)(10%)Consider the use of TCP over a wireless channel, which is much more prone to bit-level errors than a wire or optical fiber. Comment on how well suited TCP's(i)error detection mechanism, and (ii)its window-based congestion control algorithm are for such a bit-error-prone environment. Question 3: Routing Algorithms(20%) a)(10%)Consider the network shown below. Show the operation of Dijkstra's (Link State) algorithm for computing the least cost path from D to all destinations. b)(10%)Consider a node Z, which has only two beighbors -X and Y. The link cost from Z to X is 2 and the link cost from Z to Y is 3. Suppose X and Y have the distance tables shown below, which they send to Z. Complete the following table in node Z after it receives the destance tables from its neighbors X and Y: Question 4:NAT:Network Address Translation(15%) a)(5%)Suppose host A is behind a NAT, and host B has a public IP. The TCP connection between host A and B has been established. While host A sends datagram to host B and host B replies datagram to host A, what does the NAT router do? b)(10%)NATs on P2P applications: Suppose a peer A discovers through quety that peer B has a file it wants to download. Also, suppose that A and B are both behind a basic NAT which does not support any other fuctions. Is there a way to allow A to establish a TCP connection with B directly without application-specific NAT configuration? Discuss why and how to do it (if it is possible). Question 5:Subnets(10%) Considr the topology shown below. Denote the three subnets with hosts (starting clockwise at 12:00) as Networks A, B, and C. Denote the subnets without hosts as Networks D, E, and F. (a)Assign network addresses to each of these six subnets, with the following constraints: All addresses must be allocted from 214.97.254/17; Subnet A should have enough addresses to support 250 interfaces; Subnet B should have enough addresses to support 120 interfaces; and Subnet C should have enough addresses to support 120 interfaces. Of courese, subnets D, E and F should each be able to support two interfaces. For each subnet, the assignment should each be able to support two interfaces. For each subnet, the assignment should take the form a.b.c.d/x or a.b.c.d/x - e.f.g.h/y. (b)Using your answer of part (a), provide the forwarding tables (using longest prefix matching) for each of the three routers. ※ 编辑: oscarchichun 来自: 218.167.72.190 (07/12 16:25)